1. Field of the Invention
The present invention relates to a wireless communication technology.
2. Description of the Related Art
In IEEE802, specifications for a physical layer (PHY) and a data link layer (DLL), two lower layers of the OSI model, are built. Frame transmission and reception is controlled by PHY and DLL. DLL is divided into a logical link control (LLC) sublayer and a media access control (MAC) sublayer. PHY operates as an interface between MAC and a wireless transmission channel, and MAC operates as an interface between LLC and PHY.
An infrastructure network includes an access point. A wireless local area network (LAN) client transmits and receives a frame to and from the access point. The MAC unit of the access point determines a frequency channel to use and manages transmission and reception of frames to and from the wireless LAN client by using the determined frequency channel. The wireless LAN client uses the frequency channel determined by the access point to transmit and receive frames to and from the access point.
To participate in an infrastructure network, the wireless LAN client performs a scanning process for finding a network. One known technique for a scanning process is called active scan. In the active scan, a wireless LAN client transmits a probe request frame on a plurality of frequency channels included in a channel list. If there is an access point that uses the frequency channel of the probe request frame, the access point generates a probe response frame and returns the frame to the wireless LAN client. This allows the wireless LAN client to find an infrastructure network and participate in the network found.
In the IEEE802.11 specification, access to a wireless medium is provided by carrier sense multiple access/collision avoidance (CSMA/CA). In this communication protocol, the wireless LAN terminal attempts to receive any signal in a given frequency channel before initiating communication in the given frequency channel so as to determine whether there is any other terminal currently communicating (Carrier Sense). If there are not other terminals using the same frequency channel, the wireless LAN terminal starts communication (Multiple Access). If the terminal finds another terminal communicating as a result of Carrier Sense, it means that the likelihood of collision is high if transmission is attempted concurrently with the termination of communication by the other terminal. The terminal waits time of random duration after detecting termination of communication by the other terminal, and initiates transmission (Collision Avoidance). Thus, the wireless LAN terminal verifies that competition with other terminals does not occur before transmitting and receiving a frame to and from the access point.    [patent document No. 1] US2006/034315
To provide a wireless communication device with the function of connecting to an external wireless relay device operating as an access point and concurrently connecting to an external wireless terminal device (i.e., the function whereby the wireless communication device operates as a terminal and an access point), the wireless communication device preferably comprises two sets of MAC unit and PHY unit. The two sets of MAC unit and PHY unit do not basically affect each other and can operate independently of each other. Therefore, the wireless communication device provided with the two sets can use one of the sets to operate as a wireless LAN client in relation to the external wireless relay device and can use the other to operate as an access point in relation to an external wireless terminal device.
Meanwhile, the wireless communication device may be configured to use one PHY unit to connect to an external wireless relay device and an external wireless terminal device for the reason of reduction of manufacturing cost, etc. Such a wireless communication device uses the same frequency channel generated by the solitary PHY unit to transmit and receive a frame to and from an external wireless relay device and an external wireless terminal device. If the external wireless relay device changes the frequency channel to use for some reason, the air link between the wireless communication device and the external wireless relay device is disconnected. In order to restore connection with the external wireless relay device, the wireless communication device should perform a scanning process to search for the external wireless relay device. However, a scanning process requires sequentially switching the channel to use. Therefore, the frequency channel used hitherto for communication with the external wireless terminal device is changed sequentially, resulting in a disadvantage in that the connection with the external wireless terminal device cannot be maintained.